Plasma display apparatus

ABSTRACT

According to the present invention, a plasma display apparatus, which represents the luminance of one frame in accordance with a combination of plural sub-frames having luminance levels corresponding to a plurality of weighted values, additionally includes a sub-frame having a luminance level lower than the minimum gray scale level of luminance which can be represented by the number of bits in the input video data. Such plasma display apparatus turns on a desired combination of the sub-frames so as to increase the resolution of the luminance without increasing the conventional number of gray scale levels included in the input video data. In a preferred embodiment, especially when the added smaller luminance sub-frame is included in a combination of sub-frames for a low luminance area, the resolution of the gray scale of the low luminance area can be increased, and the representation of gray scale can be enhanced in a low luminance area to which the sight of a person is more sensitive.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma display apparatus, and relatesin particular to a plasma display apparatus which has an improved dataconverter and which can display a higher quality image.

2. Related Arts

A plasma display apparatus is so designed that a predetermined number ofelectrodes are formed on paired, facing substrates separated by adischarge space, that plasma discharges are generated between opposedelectrodes, that phosphors formed on the substrates are excited byultraviolet rays produced by the plasma discharges, and that apredetermined image is displayed. Furthermore, the number of dischargesin each frame is controlled, so that an image having a desired luminancecan be displayed.

FIG. 14 is a diagram for explaining a sub-frame system for aconventional three-electrodes surface discharge plasma display panel.With a three-electrode surface discharge AC plasma display, one frame isdivided into a plurality of sub-frames SF, and the luminance level isrepresented by the ratio for the numbers of the sustain discharge pulsesin the sub-frames. Provided for each sub-frame SF are: a reset periodRST, for resetting a whole panel; an address period ADD, during which acell to be displayed is discharged in accordance with desired image dataand wall charges are accumulated; and a sustain period SUS, duringwhich, for a cell wherein wall charges are accumulated when a dischargeoccurs during the address period ADD, sustain discharging is repeatedthe number of times which corresponds to the ratio for the luminancelevel of the sub-frame.

In the example in FIG. 14, one frame is formed of eight sub-frames SF1to SF8, and the ratio for the sustain discharges of the sub-frames SF1to SF8 is set as 1:2:4:8:16:32:64:128. Accordingly, the same ratio isapplied to the luminance represented by the sub-frames. Thus, thedesired luminance for one frame can be displayed in accordance with apredetermined combination of sub-frames.

Video data (or image data) input to a plasma display apparatus arenormally digital data designating gray scale of RGB colors in individualframes. Therefore, the plasma display apparatus includes a dataconverter for converting the gray scale color data of each frame intodisplay data which are constituted by combinations of sub-frames.Generally, the data converter can be implemented by a conversion table.

FIG. 15 is an example conversion table for a conventional dataconverter, and FIGS. 16A and 16B are graphs showing the characteristicof the conversion table in FIG. 15. In the examples of FIGS. 15 and 16,eight-bit input video data having 256 gray scales are converted intooutput display data which specify the ON/OFF states of the eightsub-frames SF1 to SF8, so that a luminance level of 0 to 255 isdisplayed within one frame. For example, when the gray scale level ofthe input video data is 7, the sub-frames SF1(1), SF2(2) and SF3(4) areturned on, and when the gray scale level of the input video data is 255,all the sub-frames SF1(1) to SF8(128) are turned on.

As is shown in FIG. 16A, the characteristic of the conventionalconversion table is a linear function, according to which the luminancelevel of the output display data is incremented by one when the grayscale of the input video data is incremented by one, and corresponds toa binary counter value. In accordance with the characteristic, in theconversion table in FIG. 15 the luminance levels 0 to 255 of the outputdisplay data respectively correspond to the gray scale levels of 0 to255 for the input video data. In FIG. 16B, a partially detailedcharacteristic curve is shown which corresponds to the conversion tablein FIG. 15, and in all the areas wherein the gray scale levels of theinput video data are from close to 0 up to 255, the luminance level ofthe output display data is incremented by one at the same time the grayscale of the input video data is incremented by one.

The conversion table or the conversion characteristic of theconventional data converter depicts a linear function whereby theluminance level is uniformly increased or reduced in all gray scaleareas wherein the input video data is used. However, according to thisconversion characteristic, in a low luminance area the number ofluminance levels to which the sight of a person is most sensitive isinsufficient, and the luminance level for a dark image isunsatisfactory. That is, the luminance resolution for a dark image isnot satisfactory. On the other hand, since in a high luminance area thesight of a person is not as sensitive as it is in a low luminance area,in a high luminance area there are more levels than is necessary.

Furthermore, externally input video data tend to be modulated by gammacompensation from an original video signal. The gamma compensation forsuch a video signal is performed in accordance with the gammacharacteristic of a display device, such as CRT which has more luminancelevels in a low input area. However, since the plasma display apparatushaving the conversion characteristic shown in FIGS. 15 and 16 does nothave such a gamma characteristic, for the input video data with gammacompensation, the gray scale levels can not be represented appropriatelyin a low luminance area, so luminance resolution in the low luminancearea is lost.

In addition, since the conventional data converter employs the sameconversion table or conversion characteristic shown in FIGS. 15 and 16for the three primary colors, RGB, the color balance (tint) can not bechanged. Generally, a display image having a low color temperature, suchas reddish white, is preferred in the Europe and U.S.A., while an imagehaving a high color temperature, such as bluish white, is preferred inJapan. However, the conventional plasma display apparatus can notprovide with the difference between the color temperatures. Further,because of the characteristics of the phosphors, a different luminancemay be produced for each color for a predetermined same sustaindischarge count.

SUMMARY OF THE INVENTION

It is, therefore, one objective of the present invention to provide aplasma display apparatus which can increase the gray scale levels in alow luminance area.

It is another objective of the present invention to provide a plasmadisplay apparatus which generates output display data for input videodata in accordance with an arbitrary conversion characteristic.

It is an additional objective of the present invention to provide aplasma display apparatus which can selectively display images havingdesired color temperatures.

It is a further objective of the present invention to provide a plasmadisplay apparatus having a gamma characteristic.

To achieve the above objectives, according to one aspect of the presentinvention, a plasma display apparatus, which represents the luminance ofone frame in accordance with a combination of plural sub-frames havingluminance levels corresponding to a plurality of weighted values,additionally includes a sub-frame having a luminance level lower thanthe minimum gray scale level of luminance which can be represented bythe number of bits in the input video data. Such plasma displayapparatus turns on a desired combination of the sub-frames so as toincrease the resolution of the luminance without increasing theconventional number of gray scale levels included in the input videodata.

In a preferred embodiment, especially when the added smaller luminancesub-frame is included in a combination of sub-frames for a low luminancearea, the resolution of the gray scale of the low luminance area can beincreased, and the representation of gray scale can be enhanced in a lowluminance area to which the sight of a person is more sensitive.

In another preferred embodiment, the added smaller luminance sub-frameis selectively employed to set a different conversion characteristic foreach RGB color, and an image having a desired color temperature isselectively displayed.

To achieve the above objectives, according to another aspect of theinvention, a plasma display apparatus, which represents the luminance ofone frame in accordance with a combination of sub-frames havingpredetermined luminance levels, comprises:

-   -   a data converter for converting input video data into output        data in which the ON/OFF states of the sub-frames are specified;    -   wherein the sub-frames include a smaller luminance sub-frame        having a luminance level which is lower than the minimum gray        scale level of luminance which can be represented by the number        of bits in the input video data.

In the preferred embodiments, the data converter has a conversioncharacteristic in which an increase rate of the luminance of the outputdata in a first gray scale area for input video data is lower than anincrease rate of the luminance of the output data in a second gray scalearea whose luminance is higher than that in the first gray scale area.

According to the preferred embodiments, the data converter has aplurality of conversion characteristics, and a desired conversioncharacteristic is selected in accordance with a mode set signal forselecting the conversion characteristics.

According to the preferred embodiments, the input video data aresupplied in accordance with a plurality of primary colors, and theconversion characteristics of the data converter are selectivelydetermined for each of the primary colors.

According to the preferred embodiments, the data converter converts theinput video data into output data, which includes more bits than thatused for the input video data.

In the preferred embodiments, the data converter has a conversioncharacteristics, in which an increase rate of the luminance of theoutput data in a first gray scale area for the input video data, differsfrom an increase rate of the luminance of the output data in a secondgray scale area, whose luminance is higher than that in the first grayscale area.

Furthermore, to achieve the above objectives, provided is a dataconverter, for a plasma display apparatus which represents the luminanceof one frame in accordance with a combination of sub-frames havingpredetermined luminance levels, wherein video input data are convertedinto output data in which the ON/OFF states of the plurality ofsub-frames are specified, and wherein the sub-frames include a smallerluminance sub-frame which has a luminance level lower than the minimumgray scale level of luminance which can be represented by the number ofbits in the input video data.

According to the preferred embodiments, the data converter of the plasmadisplay apparatus has a conversion characteristic in which an increaserate of the luminance of the output data in a first gray scale area forthe input video data is lower (or higher) than an increase rate of theluminance of the output data in a second gray scale area, whoseluminance is higher than that in the first gray scale area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a plasma display apparatusaccording to one embodiment of the present invention;

FIG. 2 is an exploded perspective view of a partial structure of aplasma display panel;

FIG. 3 is a diagram showing a sub-frame system according to theembodiment of the present invention;

FIG. 4 is a diagram showing the arrangement of a display data generatoraccording to the embodiment of the present invention;

FIG. 5 is a detailed diagram showing the arrangement of a data converteraccording to the embodiment of the present invention;

FIGS. 6A to 6C are graphs showing a conversion table stored in the dataconverter;

FIG. 7 is a diagram showing a conversion table for a gammacharacteristic according to the embodiment of the present invention;

FIGS. 8A and 8B are graphs showing the characteristic of the conversiontable in FIG. 7;

FIG. 9 is a diagram showing the arrangement of another display datagenerator according to the embodiment of the present invention;

FIG. 10 is a diagram showing the arrangement of an additional displaydata generator according to the embodiment of the present invention;

FIG. 11 is a diagram showing a conversion table for a gammacharacteristic when a superimposition method is employed;

FIG. 12 is a diagram showing another example data converter;

FIG. 13 is a diagram showing an example wherein tint (color temperature)is changed by using the data converter in FIG. 12;

FIG. 14 is a diagram for explaining a sub-frame system for aconventional three-electrode surface discharge AC plasma display panel;

FIG. 15 is a diagram showing an example conversion table for aconventional data converter; and

FIGS. 16A and 16B are graphs showing the characteristic of theconversion table of the conventional data converter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwhile referring to the accompanying drawings. It should be noted,however, that the technical scope of the present invention is notlimited to these embodiments.

FIG. 1 is a schematic diagram illustrating the arrangement of a plasmadisplay apparatus according to one embodiment of the present invention.The plasma display apparatus comprises: a plasma display panel PDP;drivers 30 to 36 for driving the plasma display panel PDP; and acontroller 38 for controlling the drivers 30 to 36. The controller 38includes: a display data generator 44 for receiving an input video dataVin and a dot clock Dclk, and for generating display data Dout; asynchronization controller 40, for exercising synchronous control of theplasma display apparatus in response to an externally received verticalsynchronization signal Vsync; a driver controller 42, which issynchronously controlled by the synchronization controller 40 andcontrols the drivers 30 to 36.

The driver controller 42 controls the drivers 30 to 36 in accordancewith control signals S1, S2, S3 and S4. Of these drivers, the common Xdriver 34 drives X electrodes of the plasma display panel PDP; the Yscan driver 30 sequentially drives Y electrodes to perform scanningduring the address period; the common Y driver 32 drives the Yelectrodes during the sustain period; and the address driver 36 drivesaddress electrodes in accordance with display data Dout during theaddress period.

FIG. 2 is an exploded perspective view of the plasma display panel PDP.The plasma display panel PDP is a three-electrode, surface dischargeplasma display panel having a front substrate 10 and a rear substrate20. A plurality of pairs of X electrodes 13X, and Y electrodes 13Y, eachof which is formed by laminating a transparent electrode 11 and a highlyconductive bus electrode 12, are arranged in parallel on the frontsubstrate 10, and deposited on these electrodes are a dielectric layer14 and a protective layer 15. A plurality of address electrodes A1, A2and A3, are arranged on an underlayer 21 on the rear substrate 20, and adielectric layer 22 is formed thereon. Further, on the rear substrate20, ribs 23 are formed between and along the address electrodes A1 toA3, and red, green and blue phosphors 24R, 24G and 24B, are formed onthe address electrodes A1 to A3 and the ribs 23.

In the thus arranged plasma display panel PDP, a discharge is generatedacross the entire panel between the X and Y electrodes by applying alarge reset pulse to them during the reset period RST. At the trailingedge of the reset pulse, an erasing discharge is produced by employingthe wall charges generated during the first discharge, so that the wallcharges on the dielectric layer 14 of the electrodes X and Y aredisappeared. During the address period ADD, following the reset periodRST, a scan pulse is sequentially applied to the Y electrodes 13Y, whilea write pulse is applied to the address electrodes A1 to A3 inaccordance with the display data. A discharge is thus produced betweenthe Y electrodes 13Y and the address electrodes of cells which are to beturned on, and wall charges are accumulated on the dielectric layer 14of the pertinent cell.

Then, during the sustain period SUS, a predetermined number of sustainpulses, which correspond to the luminance of the sub-frame, are appliedto all of the X and Y electrodes, so that sustain discharges areagenerated only in cells where wall charges have been accumulated and aluminance corresponding to the sub-frame is generated.

FIG. 3 is a diagram showing a sub-frame system according to thisembodiment. Shown in this example, as well as in the conventionalsub-frame system shown in FIG. 14, are a combination of sub-frames to beconverted for eight-bit input video data having 256 gray scale levels.As is apparent from the comparison of this embodiment with the examplein FIG. 14, a smaller luminance sub-frame SF0 is added to theconventional sub-frames SF1 to SF8. The sub-fame SF0 has a luminancevalue of 0.5, which is smaller than the minimum gray scale level (level1 in FIG. 3) of the luminance which can be represented by the number ofbits in the input video data.

As in the conventional example, each sub-frame SF is constituted by areset period RST, an address period ADD and a sustain period SUS. Forthe sub-frames SF1 to SF8, the number of sustain discharge pulsesgenerated during the sustain period SUS is set at a ratio of1:2:4:8:16:32:64:128. For a newly added, smaller luminance sub-frameSF0, the number of sustain discharge pulses during the sustain period isset at a ratio of 0.5 to the values for the above sub-frames. Since thenumber of sustain discharge pulses for the smaller luminance sub-frameSF0 is small, it is not very difficult to add such a sub-frame SF0during a one-frame period.

The number of smaller luminance sub-frames is not limited to one; morethan one may be used. Further, the weight of the smaller luminancesub-frame need not always be ½ of the sub-frame SF1, which has thesmallest weight, and may be set using a predetermined ratio which issmaller than the luminance of the sub-frame SF1. An optimal smallerluminance sub-frame is selected within an insertion range to a one-frameperiod.

FIG. 4 is a diagram showing the display data generator 44 according tothis embodiment. As explained while referring to FIG. 1, the displaydata generator 44 converts the input video data Vin into the outputdisplay data Dout, and in accordance with the timing for the sub-frame,provides the display data for each cell to the address driver 36. As isshown in FIG. 4, the display data generator 44 includes: a dataconverter 50, for receiving the input video data Vin for the RGB colorsand for converting the input video data Vin into display data Dout, inwhich the ON/OFF states of the sub-frames are specified; a frame memory52, in which sub-frame data obtained by conversion are stored for eachcell; and a display data provider 54, for reading the display data Doutfor each cell in consonance with the timing of the sub-frame and forproviding the display data Dout to the address driver 36 in consonancewith the timing for the scanning of the Y electrodes.

As is shown in FIG. 4, the data converter 50 converts the eight-bitinput video data Vin for each RGB color into nine-bit display data, inwhich the ON/OFF states of the sub-frames SF0 to SF8 are specified.Therefore, nine bits of display data are stored for each cell in theframe memory 52. The display data provider 54 reads, along one line ofthe Y electrodes which are scan electrodes, the display data Dout whichcorrespond to a currently driven sub-frame SFn (n=0 to 8), and thenprovides the display data Dout to the address driver 36. Simultaneously,in accordance with the timing for the scanning of the Y electrodes, theaddress driver 36 transmits the display data Dout for one line of Yelectrodes to the address electrodes.

The display data generator 44 includes a mode setting unit 56, forgenerating a mode set signal S12 in accordance with an externallyprovided mode instruction signal S10. A plurality of conversion tablescorresponding to a number of different display modes are provided forthe data converter 50, which selects a conversion table corresponding tothe mode set signal S12, which is generated in accordance with the modeinstruction signal S10, and converts the eight-bits input video signalVin into the nine-bits display data Dout, which correspond to thesub-frames.

FIG. 5 is a detailed diagram showing the arrangement of the dataconverter 50 according to this embodiment. The data converter 50receives the eight-bits input video data Vin for each RGB color, employsa conversion table corresponding to the mode set signal S12 to convertthe RGB input video data into the nine-bits display data, in which theON/OFF states of the sub-frames SF0 to SF8 are specified, and stores thedisplay data in the frame memory 52.

Since not only the eight-bits sub-frames SF1 to SF8, as in theconventional example, but also the smaller luminance sub-frame SF0,having a luminance ratio of 0.5 to the values of the smallest sub-frame,is employed for the eight-bits input video data Vin, the characteristicsof the conversion tables can be a variety of characteristics other thanthe linear function. In other words, the luminance resolutions of outputdata can be increased without changing the number of bits of the inputvideo data. Therefore, the data converter 50 stores in advanceconversion tables having a plurality of characteristics, and to performdata conversion by selecting a conversion table corresponding to a modeset signal S12.

FIGS. 6A to 6C are graphs showing the conversion tables for the dataconverter 50. In FIG. 6A is shown a linear characteristic for which anoutput level has a linear function characteristic relative to an inputlevel. This characteristic is the same as that in the conventionalconversion table in FIGS. 15 and 16. In this case, the conversion tableis the same as that shown in FIG. 15, and the converted output displaydata are data in that the smaller luminance sub-frame SF0 is always inthe OFF state. As the gray scale of the input level is changed, theluminance of the nine-bits output display data is altered by thesmallest luminance unit in accordance with the eight-bits input data.

According to the characteristic indicated by the solid line in FIG. 6B,the increase rate of the output level is low in a lower input levelarea, while the increase rate of the output level is high in a higherinput level area. This characteristic is generally called a gammacharacteristic, and is an inherent CRT display device phenomenon. Thatis, since a person can readily identify a difference in luminance in alower input level area, the gray scale resolution of the luminance isincreased to enhance the representation of a dark image. And since aperson can not easily identify a difference in luminance in a higherinput level area, the gray scale resolution of the luminance is reducedto prevent an increase in the total number of gray scale levels.

The characteristic described by a broken line in FIG. 6B corresponds tothe characteristic described by the solid line, and is a characteristicof the input video data which is provided from external viacompensation. When the input video data, which are compensated for inaccordance with the characteristic described by the broken line, areconverted by using the conversion table of the characteristic describedby the solid line, the display data having more appropriate luminancechange, in accordance with the change of the input level, can begenerated.

The conversion table for the gamma characteristic is the optimal tablefor the display of images, such as a movie, while the conversion tablefor the linear characteristic is the appropriate table for the displayof graphics, such as graphs and characters.

The characteristic shown in FIG. 6C is the S-shaped characteristic,according to which the rate of change in the output level is low both ina lower input level area and in a higher input level area, and is highin a middle input level area. When, for example, a dark image isdisplayed on a bright background image, more gray scale levels areprovided for each of the images, so that for both images the luminanceresolution can be improved. According to this conversion characteristicof the data converter, the increase rate of the luminance of the outputdata in the first gray scale area of the input video data differs fromthe increase rate of the luminance of the output data in the second grayscale area, which is higher than the first gray scale area. When theserates differ, an appropriate characteristic other than the S-shapedcharacteristic can be implemented.

Referring again to FIG. 5, since the smaller luminance sub-frame SF0 isadditionally provided, conversion tables having various characteristicscan be employed. Therefore, if an operator sets an optical mode, via themode instruction signal S10, to the mode setting unit 56, the dataconverter 50 can convert the input video data according to the optimalconversion table for the input video data. That is, when the operatorselects the optimal mode, image or video to be displayed can bedisplayed with the optimal mode.

The data converter 50 does not always require a plurality of conversiontables. A conversion table incorporating the gamma characteristic, whichconventionally is not implemented, may be permanently stored to performdata conversion. In this case, an external input mode settinginstruction is not required.

FIG. 7 is a diagram showing the conversion table according to thisembodiment in which the gamma characteristic is incorporated, and FIGS.8A and 8B are graphs showing the characteristics of the conversiontable. As is shown in the conversion table in FIG. 7, eight-bits inputdata having 256 gray scale levels, 0 to 255, are converted intonine-bits output data, in which the ON/OFF states of the sub-frames SF0to SF8 are specified. The ratio for the luminance levels of thesub-frames is, for example, 0.5:1:2:4:8:16:32:64:128.

According to the conversion table in FIG. 7, in a low input gray scalearea of 0 to 128, the luminance of output data is increased with asmaller rate of the smaller luminance sub-frame SF0. In a high inputgray scale level area of 128 to 255, the luminance of output data isincreased with a higher rate of sub-frame SF2 (a luminance level ratioof 2). For the highest input gray scale level of 255, the maximumluminance level is output to turn on all the sub-frames SF0 to SF8.

In FIG. 8A the characteristic of the conversion table in FIG. 7 is shownwhich represents the input video data and the output display data(output luminance value). The rate of increase is lower in an area C1for which the input level, 0 to 128, is also low, while the rate ofincrease is higher in an area C2 for which the input level, 128 to 255,is also high. Therefore, this characteristic corresponds to the gammacharacteristic in FIG. 6B.

FIG. 8B is a detailed graph showing the change in the luminance of theoutput display data relative to the quantization level (skipping digitallevel) of eight-bits input video data. In an area C1 for which the inputlevel, 0 to 128, is low, the rate of increase is so low that theluminance of the output display data is increased at the luminance ratioof the smaller luminance sub-frame SF0. In an area C2 for which theinput level, 128 to 255, is high, the increase rate is so high that theluminance of the output display data is increased at the luminance ratio2 of the sub-frame SF2.

When the input level area is divided into three or more portions and,beginning at the low level area, the smaller luminance sub-frame SF0 isappropriately employed to gradually advance the rate of increase, acharacteristic of the conversion table in FIG. 7 can be closer to thegamma characteristic in FIG. 6B. Further, the characteristic curve canbe smoothed by adding more smaller luminance sub-frames. Here, it ispreferable that the maximum number of smaller luminance sub-frames beemployed within a range corresponding to a one-frame period.

FIG. 9 is a diagram showing another display data generator according tothe embodiment. In FIGS. 4 and 5, upon receiving a mode instructionsignal S10 the display data generator 44 performs a mode settingprocess, and to perform data conversion, employs a conversion tablecorresponding to the designated mode. In the example in FIG. 9, however,a mode determiner 57 analyzes video data Vin to determine whether thosedata are image data for characters and graphs (images or graphsgenerated by a computer), or image data for natural images, such asphotographs, and generates an optimal mode set signal S12 for the image.Thereafter, to perform data conversion the data converter 50 employs aconversion table corresponding to the mode set signal S12. For example,the conversion table incorporating the linear characteristic in FIG. 6Ais selected for characters and graphs, while the conversion tableincorporating the gamma characteristic in FIG. 6B is selected forphotographs and movies.

The mode determiner 57 can determine, to a degree, the original inputvideo data image type by analyzing the data for a plurality ofcontiguous frames, or by preparing and analyzing a histogram for eachgray scale level included in the input video data for the frames.

FIG. 10 is a diagram illustrating the arrangement of another displaydata generator according to this embodiment. In the example in FIG. 10 adata converter 50 includes a superimposition method to prevent theoccurrence of a so-called false color outline. During the process inwhich the luminance is represented by using a combination of thesub-frames in FIG. 3, a false color outline occurs when, for example,images for the luminance levels of 128 and 127 are alternatelydisplayed. That is, the luminance level for 128 can be generated byturning on the sub-frame SF8, while the luminance level for 127 can beproduced by turning on the sub-frames SF1 to SF7. Therefore, if thesub-frames SF0 to SF8 are arranged in order and when the luminancelevels 128 and 127 are repeated alternately, the display process forsequentially turning on the sub-frame SF8 and the sub-frames SF1 to SF7is also repeated. Therefore, flicker occurs and a false color outline isgenerated when an image is moved.

To prevent the occurrence of this phenomenon, a superimposition methodhas been proposed. According to this method, a sub-frame having a highluminance level is divided, and the sub-frames in a single frame arearranged so that sub-frames having high luminance levels are notadjacently positioned. In the example in FIG. 10, the eight-bits inputvideo data Vin, with which 256 gray scale levels are provided, areconverted into output data for a sub-frame combination consisting ofsmaller luminance sub-frame SF0 and sub-frames SF1 to SF10.

The luminance ratio for the ten sub-frames SF1 to SF10 is1:2:4:8:16:32:32:32:64:64. That is, the sub-frame SF7, which has aluminance level of 64, is divided into sub-frames SF7 and SF8, each ofwhich has a luminance level of 32, and the sub-frame SF8, which has aluminance level of 128, is divided into sub-frames SF9 and SF10, each ofwhich has a luminance level of 64.

FIG. 11 is a diagram showing a conversion table incorporating the gammacharacteristic which uses the superimposition method. The characteristicfor this conversion table is the same as that shown in FIGS. 8 and 9.That is, when the superimposition method is used, the conversion tablein FIG. 7 can be changed to the conversion table in FIG. 11. Then,according to the conversion table in FIG. 11, in a low input level areathe luminance of the output data is changed by the luminance ratio 0.5for the smaller luminance sub-frame, while in a high input level areathe luminance of the output data is changed by the luminance ratio 2.0for the sub-frame SF2.

It should be noted that, in accordance with the conversion table in FIG.11, the sub-frames SF7 (32) and SF8 (32) are simultaneously turned onwhen the sub-frame SF7 (64) is turned on in FIG. 7. Similarly, when thesub-frame SF8 (128) is turned on in FIG. 7, the sub-frames SF9 (64) andSF10 (64) are simultaneously turned on in FIG. 11. When thesuperimposition method is used, the sub-frames SF0 to SF10 in FIG. 11are randomly arranged.

FIG. 12 is a diagram showing another data converter. The data converter50 in FIG. 12 can select a conversion table, as needed, for each of thethree primary, RGB, colors. When the appropriate conversion table isselected for each color, an image can be displayed for which a desiredtint (color temperature) is used. Further, the optimal conversion tablecan be selected in accordance with the characteristic of a phosphorwhich corresponds to a color.

For RGB, the data converter 50 in FIG. 12 includes data converters 50R,50G and 50B, each of which receives eight-bits input video data Vin.Each data converter converts the received input video data Vin intodisplay data Dout(R), Dout(G) or Dout(B) for a nine-bits sub-frames byusing an appropriate conversion table which is selected from among threetables provided for this purpose in accordance with a mode set signalS12R, S12G or S12B.

FIG. 13 is a diagram showing an example for which the data converter 50in FIG. 12 is used to change a tint (a color temperature). As is shownin FIG. 13, the conversion tables provided for the data converter 50 area first conversion table T1, used to provide a luminance of 80% relativeto the maximum input level; a second conversion table T2, used toprovide a luminance of 90% relative to the maximum input level; and athird conversion table T3, used to provide a luminance of 100% relativeto the maximum input level. In this case, the second conversion table T2is employed for input data for red R and green G, and the thirdconversion table T3 is employed for input data for blue B.

As a result, a synthesized image is slightly bluish which is a highercolor temperature. Therefore, an image which is preferred by Japanesepeople can be displayed by using a combination of the above conversiontables.

On the other hand, when the third conversion table T3 is employed forthe input data for red R and the second conversion table T2 or the firstconversion table T1 is employed for the input data for green G and blueB, a reddish image having a lower color temperature, which is preferredby Western people, can be displayed.

The three types of conversion tables can be tables all of which have thegamma characteristic, but have lower luminance, middle luminance andhigher luminance for all input levels. In this case, for video datawhich are obtained by gamma compensation, an image having a high or lowcolor temperature can be selectively displayed, while a high luminanceresolution in a low gray scale area is maintained.

As is described above, since an appropriate conversion table can beselected for each color, an image having a desired tint (colortemperature) can be displayed.

The data converter of the invention converts the input video data intodisplay data which is constituted by sub-frames, including a smallerluminance sub-frame which can display an image at a luminance levellower than the minimum gray scale level generated by the input data. Thecharacteristics of the conversion tables are not limited to thosedescribed above, and can be variously modified. Further, when adifferent conversion table is employed for each color, a greenish imagecan be displayed by adjusting a deviation in a color temperature curvegraph in the positive direction, or a reddish image can be displayed byadjusting the deviation in the negative direction, without anyrestrictions being imposed relative to color temperatures.

According to the present invention, in accordance with the gammacharacteristic, the input video data can be converted into display datain which the ON/OFF states of sub-frames are specified, an image, aphotograph or a movie, can be optimally displayed, and the image qualitycan be improved.

Furthermore, according to the present invention, the input video datacan be converted into display data accounting to an optimal conversioncharacteristic for an input image, and the image quality can beenhanced.

In addition, according to the present invention, an image can bedisplayed at an optimal color temperature, and the color temperature ofan image to be displayed can be selected as needed.

The protective scope of the present invention is not limited to theabove embodiment, but can cover the invention as cited in the includedclaims for the invention and its equivalents.

1. A plasma apparatus, which represents the luminance of one frame inaccordance with a combination of sub-frames having predeterminedluminance levels, comprising: a data converter to convert input videodata into output data of each pixel into output data in which the ON/OFFstates of the sub-frames are specified; wherein a number of gray scalelevels of the output data is greater than a number of gray scale levelsof the input video data, and the sub-frames include a smaller luminancesub-frame having a luminance level which is lower than the minimum grayscale level of luminance which can be represented by the number of bitsof the input video data.
 2. A plasma display apparatus according toclaim 1, wherein said data converter has a plurality of conversioncharacteristics, and a desired conversion characteristic is selected inaccordance with a mode set signal to select said plurality of conversioncharacteristics.
 3. A plasma display apparatus according to claim 1,wherein said input video data are supplied in accordance with aplurality of primary colors, and said conversion characteristics of saiddata converter are selectively determined for each of said primarycolors.
 4. A plasma display apparatus according to claim 1, wherein saiddata converter has a conversion characteristic in which an increase rateof the luminance of said output data, in a first gray scale area forsaid input video data, differs from an increase rate of said luminanceof said output data, in a second gray scale area, whose luminance ishigher than said first gray scale area.
 5. A data converter used with aplasma display apparatus which represents the luminance of one frame inaccordance with a combination of sub-frames having predeterminedluminance levels, wherein video input data of each pixel is convertedinto output data in which the ON/OFF states of the plurality ofsub-frames are specified, and wherein a number of gray scale levels ofthe output data is greater than a number of gray scale levels of theinput video data, and the sub-frames include a smaller luminancesub-frame which has a luminance level lower than the minimum gray scalelevel of luminance which can be represented by the number of bits in theinput video data.
 6. A data converter according to claim 5, wherein aconversion characteristic of the data converter is that an increase rateof the luminance of the output data, in a first gray scale area for thevideo input data, is lower (or higher) than an increase rate of theluminance of the output data, in a second gray scale area, whoseluminance is higher than that in the first gray scale area.
 7. A drivingmethod for a plasma display apparatus which represents the luminance ofone frame in accordance with a combination of sub-frames havingpredetermined luminance levels, comprising: converting video input dataof each pixel into output data in which the ON/OFF states of theplurality of sub-frames are specified; wherein a number of gray scalelevels of the output data is greater than a number of gray scale levelsof the input video data, and the sub-frames include a smaller luminancesub-frame which has a luminance level lower than the minimum gray scalelevel of luminance which can be represented by the number of bits in theinput video data.